Many repetitive and mechanical manufacturing operations, for example, welding, are increasingly being automated through the use of programmable robots. Thus, for example, robotic welding systems are experiencing widespread use. In robotic welding systems, repeated welding operations may be programmed into a robot controller for automatic operation.
During certain welding operations, it is often necessary to weld large plates together in a substantially perpendicular relationship. For plates that are welded in such a manner, a fillet weld is often used to join the plates. A fillet weld is essentially right-triangular in cross-section, with each side of the right triangle that contacts the one of the two plates referred to as a leg. The size of a fillet weld is often characterized in terms of leg length. The leg length is advantageously selected such that the fillet weld provides a bond of adequate strength between the two plates.
The controller in a robotic welding system that performs fillet welds uses leg length information to determine the wire feed rate, the weave size and/or the weld velocity travel which are then used to control the robot and the associated welding tool. For example, if a leg length of 15 mm is utilized, the controller may use that information in connection with other parameters to control the travel velocity of the robotic welder.
A problem arises, however, when such robotic devices are employed in welding applications that have fixturing tolerances. Specifically, due to varying tolerances in the manufacture of plates, there is often a gap between the two plates when they are fixtured for welding purposes. The welding process must often be adjusted to account for the gap. For example, once the width of the gap exceeds a certain threshold value, the leg length must be extended to ensure the strength of the fillet weld. By extending the leg length, more well material is required to form the fillet weld. Programmed robotic devices, however, are typically designed for predefined operation and do not typically self-adjust to compensate for gap variations from piece to piece.
Another drawback associated with the gap between the two plates is that a quantity of weld material can be lost to the gap from the fillet weld. In particular, as the fillet weld is being formed, molten weld material flows from the fillet weld into the gap between the two plates thereby requiring additional weld material in order to create the fillet weld. As the width of the gap between the plates increases, the amount of weld material lost to the gap increases. It has also been observed that the amount of weld material lost in the gap depends to some degree on the orientation of the weld, or in other words, whether the weld is an overhead weld, a vertical weld or a horizontal weld.
The increased weld material requirements resulting from the gap between the plates typically does not create a significant problem in manual welding operations. In manual operations, the welder may continuously observe the weld and the gap and determine on an ad hoc basis whether to use more welding material. However, in preprogrammed robotic welding operations, if no feedback information pertaining to the gap is provided, the controller will not adjust the welding parameters to account for the gap.
Adaptive welding processes have been developed that use a microprocessor to analyze sensor information and control a number of weld parameters based on the sensor information. Typically, adaptive welding processes have been used to control the position of a welding torch in order produce a weld that follows an irregular welding path. One such device is described in U.S. Pat. No. 4,621,185 to Brown. Such devices, however, do not account for increased weld material requirements due to the appearance of a gap between plates of a fillet weld.
Other adaptive welding processes use feedback from a sensor to observe the weld spot or weld bead during the welding operation and adjust welding parameters based on the feedback. Processes of this nature would appear to have some effectiveness in accounting for variations in welding requirements caused by a gap between two plates. A device that uses such a process is described in U.S. Pat. No. 4,724,302. These processes, however, require that a sensor constantly gather data, for example visual data, from an active welding operation at the point of the weld. That requirement is undesirable because the volatility of the welding operation may damage the sensor, or cause the sensor to provide inaccurate data.
What is needed therefore is an apparatus and method for automatically adjusting the weld parameters to account properly for the effects of a gap between two plates to be welded together without requiring continuous information regarding the weld spot or weld bead. A further need exists for such a method that also accounts for the weld material lost in the gap during the welding operation.